The present invention relates to a fuel in microemulsion form, particularly for supplying Diesel engines, and to a method for preparing it.
Emulsions or microemulsions of petroleum products and water in which particular surfactants or mixtures of surfactants are used are known in the art.
For example, U.S. Pat. No. 3,876,391 discloses microemulsions of hydrocarbon products with water in which a mixture of surfactants is used which is constituted by a first surfactant which is soluble in the oil phase and a second surfactant which is soluble in the aqueous phase, to which a further water-soluble additive, for example an amide, an alkanolamine, a polyamine or an aldehyde, is added.
U.S. Pat. No. 4,465,494 discloses microemulsions of liquid fuels and water which contain an alcohol or an amine and, as surfactant, a salt of an alkylphenoxyalkanoic acid.
A fuel emulsified with water is disclosed in EP-630,398 and is obtained by mixing the compounds in a static mixer in particular pressure and temperature conditions in the presence of a mixture of surfactants constituted by sorbitan oleate, a polyalkylene glycol and an alkylphenol ethoxylate.
In general, the use of surfactants or other additives such as the ones mentioned above can entail problems both because they can be inherently corrosive with respect to the devices with which they come into contact and because toxic by-products can form during combustion.
Moreover, on the basis of the experience of the Applicant, emulsions of liquid fuels and water prepared according to conventional methods by adding appropriate surfactants generally entail stability problems even in optimum storage conditions, so that after a certain time an at least partial separation of the phases is observed which entails many drawbacks during the combustion process owing to the non-uniformity of the supplied fuel.
EP-372,353 by the same Applicant discloses a stabilized emulsion of a fuel, particularly a fuel for Diesel engines, and water, with the addition of a product which acts as a lubricant and antifreeze, for example sorbitol monoleate. The stabilized emulsions of fuel are prepared by using a turbine-effect emulsifier such as the one disclosed in EP-124,061 in the name of this same Applicant.
Applicant has noted that in some cases, particularly in case of use of low-density fuels, the preparation of the fuels as disclosed in EP-124,061 and EP-372,353 entails a relatively high energy expenditure and a reduction in the productivity of the system, if one seeks high productivity and stability.
A method with improved efficiency for forming the emulsion of a fuel is disclosed in co-pending EPA No. 00121331.3 and corresponding co-pending U.S. application Ser. No. 09/684,649 filed on Oct. 10, 2000 by the same Applicant, included herein by reference. This method requires the use of an apparatus having a particular geometry.
The above applications disclose an apparatus and a method for forming stabilized atomized microemulsions from different liquids which are normally immiscible; the apparatus comprises a primary chamber and a sequence of at least two cavitation chambers arranged in succession, means for feeding primary and secondary fluids into the primary chamber, and means for the exit of the formed microemulsion from the last cavitation chamber, the primary chamber and the cavitation chambers being fluid-connected to each other by way of fluid passage means which are adapted to produce a velocity of the fluids, during passage through the passage means, which gradually increases from the primary chamber toward the last cavitation chamber. The method according to the above applications comprises the stage of premixing the primary fluid with the secondary fluid, followed by the passage of the premix of fluids through a succession of steps of flow at a higher velocity alternated with steps of flow at a lower velocity, the higher flow velocities gradually increasing from the first higher-velocity step to the last higher-velocity step.